Request for Comments: 6784
Category: Standards Track
Kerberos Options for DHCPv6
This document defines four new options for the Dynamic Host Configuration Protocol for IPv6 (DHCPv6). These options are used to carry configuration information for Kerberos.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.
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Table of Contents
1. Introduction ....................................................3 2. Conventions Used in This Document ...............................4 3. Kerberos Options ................................................4 3.1. Kerberos Principal Name Option .............................4 3.2. Kerberos Realm Name Option .................................5 3.3. Kerberos Default Realm Name Option .........................6 3.4. Kerberos KDC Option ........................................6 4. Client and Server Operation .....................................7 4.1. KDC Discovery for a Client .................................8 5. IANA Considerations .............................................8 6. Security Considerations .........................................9 7. Acknowledgments .................................................9 8. References .....................................................10 8.1. Normative References ......................................10 8.2. Informative References ....................................10 Appendix A. An Example of the Operation of the Client .............11
Kerberos Version 5 [RFC4120] is a trusted third-party authentication system. Each organization wishing to use Kerberos establishes its own "realm", and each client is registered as part of that realm. At least one Key Distribution Center (KDC) is required for the operation of a Kerberos realm.
When a client wishes to communicate with, and be authenticated to, a Kerberos application server (also a client of the KDC), the client identifies itself, and its realm, to the KDC and acquires a credential from the KDC. The client then presents the credential to the Kerberos application server, which can use the credential to authenticate the client. The client needs to know at least one IP address for a KDC in order to initiate this process.
One example of the application of this protocol is as follows. A student might want to use a shared, public workstation, one that is not configured for Kerberos. If there is a mechanism for the workstation to obtain a realm name and IP address for a KDC, then a student need only input a user-id and pass phrase to be able to use Kerberos.
The Kerberos V5 specification [RFC4120] defines the use of DNS SRV records [RFC2782] for KDC discovery. Some systems, such as industrial systems, do not use DNS. Such systems already have their own name spaces and their own name resolution systems, including preconfigured mapping tables for devices, and do not use Fully Qualified Domain Names. However, many of these systems do use DHCP.
Adding a DNS server to such systems may decrease the reliability of the system and increase the management cost. In such an environment, another mechanism is needed to provide an IP address for the KDC. For the PacketCable Architecture [PCARCH], RFC 3634 [RFC3634] defines the KDC Server Address sub-option for the DHCPv4 CableLabs Client Configuration option. However, a mechanism is still needed to provide a realm name and an IPv6 address -- one that does not depend on any external architecture.
This document defines a Kerberos option for DHCPv6 that provides a realm name and/or a list of KDC IP addresses. This option does not replace or modify any of the existing methods for obtaining this information.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].
3. Kerberos Options
This document defines four DHCPv6 configuration parameters for Kerberos.
Kerberos Principal Name Option
Kerberos Realm Name Option
Kerberos Default Realm Name Option
Kerberos KDC Option
This section describes the format of each option and the usage of each field in that option.
These options, except for the Kerberos KDC Option, MUST NOT appear more than once in a DHCPv6 message.
3.1. Kerberos Principal Name Option
The Kerberos Principal Name Option carries the name of a Kerberos principal. This is sent by the client to the DHCPv6 server, which MAY use it to select a specific set of configuration parameters, either for a client or for a Kerberos application server.
The format of the Kerberos Principal Name Option is:
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | OPTION_KRB_PRINCIPAL_NAME | option-len | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : : : principal-name : : (variable length) : : : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ o option-code (16 bits): OPTION_KRB_PRINCIPAL_NAME (75)
- option-len (16 bits): length of the principal-name field.
3.2. Kerberos Realm Name Option
The Kerberos Realm Name Option carries a Kerberos realm name. A DHCPv6 client uses this option to specify to a DHCPv6 server which realm the client wants to access.
The format of the Kerberos Realm Name Option is:
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | OPTION_KRB_REALM_NAME | option-len | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : : : realm-name : : (variable length) : : : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ o option-code (16 bits): OPTION_KRB_REALM_NAME (76)
- option-len (16 bits): the length of the realm-name field in octets.
3.3. Kerberos Default Realm Name Option
The Kerberos Default Realm Name Option is used to specify a default realm name for the Kerberos system. A DHCPv6 server uses this option to specify the default realm name to both clients and Kerberos application servers.
The option-code of this option is OPTION_KRB_DEFAULT_REALM_NAME (77). The format and usage of the option-len and realm-name fields are identical to those for the Kerberos Realm Name Option.
3.4. Kerberos KDC Option
The Kerberos KDC Option is used to provide configuration information about a KDC.
The format of the Kerberos KDC Option is:
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | OPTION_KRB_KDC | option-len | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Priority | Weight | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Transport Type| Port Number | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | | KDC IPv6 address +---------------+ | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : : : : realm-name : : (variable length) : : : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ o option-code (16 bits): OPTION_KRB_KDC (78)
- option-len (16 bits): 23 + the length of the realm-name field in octets.
- Priority (16 bits): see the description of the Weight field.
- Weight (16 bits): the Priority and Weight fields provide a hint to the client as to which KDC to select. The usage of the Priority and Weight values MUST follow the specification for DNS SRV [RFC2782].
- Transport Type (8 bits): The Transport Type specifies the transport protocol used for Kerberos. Kerberos [RFC4120] defines UDP and TCP transports. Exchanges over TCP are further described in [RFC5021], while the transport of Kerberos over Transport Layer Security (TLS) is described in [RFC6251].
The transport type is defined below.
Value Transport Type ---- -------------- 0 Reserved 1 UDP 2 TCP 3 TLS 4-254 Unassigned 255 Reserved
- Port Number (16 bits): the port number on which the KDC listens.
- KDC IPv6 address (128 bits): the IPv6 address of the KDC.
4. Client and Server Operation
This section describes the operations of the client and server. It assumes that the client has been configured with a principal name.
If a client requires a realm name, the client sends a DHCPv6 Option Request Option (ORO) specifying the Kerberos Default Realm Name Option. The DHCPv6 server responds with a Reply message containing a Kerberos Default Realm Name Option.
If a client requires configuration parameters for a KDC, the client sends a DHCPv6 ORO specifying the Kerberos KDC Option. The client MAY include a Kerberos Principal Name Option. The client MAY include a Kerberos Realm Name Option.
The DHCPv6 server replies with one or more sets of configuration parameters for a Kerberos KDC. If the client has specified either a
Kerberos Principal Name Option or a Kerberos Realm Name Option, then the DHCPv6 server MAY use those parameters to select specific sets of configuration parameters.
Where the server replies with more than one set of configuration parameters, the usage of the Priority and Weight fields by the client MUST follow the specification for DNS SRV [RFC2782].
The client MAY include other options with data values as hints to the DHCPv6 server about parameter values the client would like to have returned; this is specified in Section 18.1.5 of RFC 3315 [RFC3315].
4.1. KDC Discovery for a Client
When a client implements both the DNS method defined by Section 22.214.171.124 of [RFC4120] and the DHCP method defined by this document, the choice of method is determined by local policy. The administrator of the realm usually defines the method as part of the configuration of the client before the client is installed.
When no criteria have been specified and the client could get the Kerberos information from either the DNS server or the DHCPv6 server, then the information from DNS SHOULD be preferred.
5. IANA Considerations
IANA has assigned four option codes from the DHCPv6 Option Codes registry for the following:
IANA has created the Kerberos Message Transport Types sub-registry, under the Kerberos Parameters registry. The initial entries are described in Section 3.4.
The assignment of future entries is by "IETF Review" policy as described in BCP 26 [RFC5226]. Per that policy, a document specifies the symbolic name of such entries, which are assigned numeric codes by IANA once publication is approved.
6. Security Considerations
DHCPv6 messages can be modified in transit. If an adversary modifies the response from a DHCPv6 server or injects its own response, a client may be led into contacting a malicious KDC. Both cases are categorized as a Denial-of-Service (DoS) attack. However, a malicious KDC does not know the shared key and so is unable to proceed any further with the exchange. If a client receives a response from such a KDC, the client can use the shared key to detect that the message originates from a malicious KDC.
A shared, unconfigured workstation may obtain its KDC information, and default realm, via DHCPv6. Such a workstation may not have a host or other service key, and thus it may be unable to validate the Ticket-Granting Ticket issued by the KDC. A modified DHCPv6 response would then result in the workstation talking to a malicious KDC, and the workstation would not be able to detect that this has happened. This in turn could allow access by unauthorized users.
Kerberos information may be manually configured on the client before requesting information from DHCPv6. Manual configuration of the device SHOULD be preferred to configuration via the DHCPv6 server.
The authors are very grateful to Nobuo Okabe and Shigeya Suzuki. They contributed the explanation as to why DNS is inappropriate for some industry networks. Ted Lemon made many suggestions to improve DHCP aspects of this specification. Ken'ichi Kamada and Yukiyo Akisada contributed to the initial work on this document. Tom Petch helped to improve the readability of this document. The authors also thank Jeffrey Hutzelman, Kazunori Miyazawa, Kensuke Hosoya, Nicolas Williams, Nobumichi Ozoe, Sam Hartman, and Stephen Farrell. They made valuable comments and suggestions.
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, February 2000. [RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003. [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The Kerberos Network Authentication Service (V5)", RFC 4120, July 2005. [RFC5021] Josefsson, S., "Extended Kerberos Version 5 Key Distribution Center (KDC) Exchanges over TCP", RFC 5021, August 2007. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.
8.2. Informative References
[PCARCH] CableLabs, "PacketCable 1.0(TM) Architecture Framework Technical Report", December 1999, <http://www.packetcable.com/downloads/specs/ pkt-tr-arch-v01-991201.pdf>. [RFC3634] Luehrs, K., Woundy, R., Bevilacqua, J., and N. Davoust, "Key Distribution Center (KDC) Server Address Sub-option for the Dynamic Host Configuration Protocol (DHCP) CableLabs Client Configuration (CCC) Option", RFC 3634, December 2003. [RFC6251] Josefsson, S., "Using Kerberos Version 5 over the Transport Layer Security (TLS) Protocol", RFC 6251, May 2011.
Appendix A. An Example of the Operation of the Client
When no criteria have been specified and the client could get the Kerberos information from either the DNS server or the DHCPv6 server, then the information from DNS SHOULD be preferred. The following is an informational guide for the client in such an environment.
No Resp. or +------------+ DNS Info. +-----------+ No Resp. Start --> | Ask DHCP(1)| ---------> | Ask DNS(3)| ------> +------------+ +-----------+ Terminate(4) / \ \ Only KRB / \ DNS and \ KRB Info. Info. / \ KRB Info. \ / \ \ | | | | V | V No Ans. +-----------+ KRB Info. V Use Info. <-------- | Ask DNS(6)| ---------> Use Info. from DHCP +-----------+ from DNS (2), (7) (5), (8)
Resp.: Response Info.: Information KRB : Kerberos
- Initially, the client requests both DNS and Kerberos information from the DHCPv6 server.
- If the DHCPv6 server replies with Kerberos information and not with DNS information, then the client uses that information.
- If the DHCPv6 server does not reply or replies with only DNS information, then the client requests Kerberos information from the DNS server.
- If the client gets no response or no Kerberos information from the DNS server, then the client terminates the process.
- If the client gets Kerberos information from the DNS server, then the client uses that information.
- If, as the result of (1), the DHCPv6 server replies with both DNS and Kerberos information, then the client requests Kerberos information from the DNS server.
- If the client gets no response from the DNS server, then the client uses the Kerberos information from the DHCPv6 server.
- If, as the result of (6), the DNS server replies with Kerberos information, then the client uses the information from the DNS server and not that from the DHCPv6 server.
Shoichi Sakane Cisco Systems 9-7-1 Akasaka Minato-ku, Tokyo 107-6227 Japan
firstname.lastname@example.org Masahiro Ishiyama Toshiba Corporation 1, Komukai-toshiba-cho, Saiwai-ku, Kawasaki, Kanagawa 212-8582 Japan EMail: email@example.com